The present invention relates to a solid state image sensing device.
When a picture moving at a high speed is sensed by a solid state image sensing device, the solid state image sensing device is activated by an electronic shutter operation. In this electronic shutter operation, a pulse is applied to the semiconductor substrate to extract signal charges accumulated at respective pixel portions to the semiconductor substrate side. In this case, however, where the pixel dimensions decrease with the advance of microminiaturization, since the relative change in the overflow channel potential at the pixel portion becomes small relative to the potential change of the semiconductor substrate, it has become difficult to extract the signal charges from the pixel portions to the substrate side. To improve this problem, it has been necessary to increase the impurity concentration of the semiconductor substrate.
When the impurity concentration of the semiconductor substrate increases, however, a difference in impurity concentration between the semiconductor substrate and wells formed on the surface of the semiconductor substrate becomes small, with the result that when a pulse is applied to the semiconductor substrate, a punch through phenomenon is inevitably generated in a source follower circuit of the output section of the image sensing device. This punch through phenomenon is explained with reference to the attached drawing.
FIG. 1 shows a structure of an output section in a solid state image sensing device related to the present invention. In the drawing, a p-type well 12 is formed in the surface of an n-type semiconductor substrate 11, and the output section is formed in this p-type well 12. Further, the p-type well 12 is connected to the ground.
Further, on the surface portion of the p-type well 12, an N-channel MOS transistor N2 having n-type diffusion layers 15 and 16 formed as drain and source regions thereof is formed. This N-channel MOS transistor N2 is used as a transistor for outputting a signal charge. Further, the gate OG of this N-channel transistor N2 is connected to an n-type diffusion layer 14 and a source of a resetting N-channel transistor N1.
Between the n-type diffusion layer 16 (which corresponds to the source region of the outputting N-channel transistor N2) and the ground potential terminal 17, an N-channel transistor N3 used as a load transistor is formed.
This N-channel transistor N3 is of depletion type, in which n-type impurities are implanted in a channel region thereof. The N-channel transistor N3 has an n-type diffusion layer 16 as a drain and an n-type diffusion layer 17 as a source. Further, a ground potential terminal and a p.sup.+ -type diffusion layer 18 are connected to the n-type diffusion layer 17 of the source region of this N-channel transistor N3.
In the output section formed as described above, when the impurity concentration of the n-type semiconductor substrate 11 is increased to extract the signal charge, the difference in impurity concentration between the substrate 11 and the p-type well 12 becomes small. Accordingly, when a pulse is applied to the n-type semiconductor substrate 11 during the electronic shutter operation, the p-type well 12 is depleted, so that a punch through current I flows from the n-type diffusion layer (the grounded source) 17 of the load N-channel transistor N3 to the n-type semiconductor substrate 11. As a result, since the potential of the n-type diffusion layer 17 rises beyond the ground potential, the S/N ratio of the signal outputted by the output N-channel transistor N2 deteriorates.